DISSECTION OF THE BRAIN, OR ENCEPHALON. 247 



it from the posterior extremities of the cerebral hemispheres. It is 

 traversed in the antero-posterior direction by two shallow grooves, 

 which divide it into a middle and two lateral lobes. The middle 

 lobe is the smallest, and is known as the vermiform lobe. When 

 followed forwards the vermiform lobe is seen to be reflected round 

 the anterior aspect of the cerebellum to gain its lower surface at 

 the roof of the 4th ventricle, and it terminates by a blunt end 

 about the middle of this surface. When followed posteriorly the 

 vermiform lobe behaves in the same way, terminating at the roof of the 

 4th ventricle by a blunt end opposed to the first. These reflected 

 portions have a distinct resemblance to tw r o caterpillars, and they may 

 be distinguished as the anterior and posterior vermiform processes. The 

 anterior vermiform process is adherent to the valve of Vieussens. Each 

 lateral lobe is joined on its inferior aspect by three bundles of nerve 

 fibres, which are termed the peduncles. The posterior peduncle is the 

 termination of the restiform body, the middle peduncle is the reflected 

 extremity of the pons, and the anterior peduncle passes forwards beneath 

 the corpora quadrigemina. 



Besides the grooves which divide the cerebellum into its lobes, 

 numerous smaller fissures occur over its surface, and divide the lobes 

 into folia, or leaflets. The arrangement of these leaflets will be made 

 much more evident by making an antero-posterior vertical section, at or 

 near the mesial plane of the organ. The peduncles are to be cut as 

 they enter the lower face of the lateral lobe, and the anterior vermiform 

 process is to be carefully separated from the valve of Vieussens with the 

 scalpel. This will enable one half or a little more of the cerebellum to 

 be removed after the manner of Plate 35. 



The cerebellum contains both grey and white matter. The white 

 matter forms a large mass in the interior, and from this mass large 

 plates are given off towards the surface. From these primary plates 

 proceed more numerous smaller secondary plates, and these again 

 detach small terminal plates which end in the surface folia. In con- 

 sequence of this disposition of the white matter, it presents on vertical 

 section a strikingly aborescent appearance, to which the term arbor 

 vitae is applied. The nerve fibres of the white matter are for the most 

 part directly continuous with the peduncles ; but some are proper to the 

 organ, and connect different points of the grey matter. 



The grey matter of the cerebellum invests the core of white matter 

 within each leaflet, and also extends across the bottom of the fissures 

 between adjacent leaflets. It consists of two strata; an outer grey 

 layer, and an inner rust-coloured layer. 



The Fourth Ventricle (Plates 35 and 36). This is a space between 

 the cerebellum above, and the medulla and pons below. Its boundaries 

 are as follows : Its floor is formed by the medulla and pons ; its roof 



